On average, transits of Mercury occur about 13 or 14 times every 100 years. The last one took place in 2006, and the next one won’t occur until November 11, 2019. After that, we won’t see another until 2032.

Why so uncommon? For a transit to occur, the sun, Mercury, and Earth all have to line up directly. But Mercury’s orbit is inclined by about 7 degrees compared with Earth’s. So there are only two spots where the two planets could conceivably line up with the sun — the places where Mercury crosses the Earth’s orbital plane…

Earth lines up with these intersection spots around May 8 and November 10 each year, give or take a few days. If Mercury, which takes 88 days to orbit the sun, is also wandering through at the same time, a transit occurs. This happens once every seven or eight years.

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